Transactions of the Geological Society, 1st series, vol. 2/On the Mineralogy of St. David's
By John Kidd, M.D. Prof. Chem. in the University of Oxford, M.G.S.
THE following notes are arranged under separate heads, descriptive of particular points of the country adjacent to St. David's, which were visited during a stay of a few days at that place in the summer of 1811: and for obvious reasons, such points were selected as might from description be easily referred to by others inclined to examine the same ground.
No order has been adopted in the distribution of these heads than was required for the convenience of description; for there did not on the spot, and to an unprejudiced observer, appear to be any obvious and natural chain of connexion between the several points here described.
The country round St. David's, when viewed from an eminence, presents the appearance of an extensive uneven plain, interspersed with numerous detached hills or rocky summits of an irregularly conical shape. The rocks which constitute these hills bear no marks of regular stratification; rarely support even a slight degree of vegetation; and when compared with the surrounding surface, appear as so many nuclei, about which is arranged a very curiously diversified series of highly inclined strata of a kind of slate. The constituent parts of these insulated rocks are felspar and hornblende; and the general character of them is crystalline: but the felspar rarely if ever occurs in distinct crystals; and even the hornblende, though usually the most accurately defined of the two, is sometimes not discernible from the felspar. Again, though the mass often appears upon the whole to have been of chemical origin, yet at the same time it is, partially, of a structure to the eye decidedly mechanical; and in some instances the character is so extremely equivocal as to leave the judgment in a very difficult state of suspense. The predominating colour of these crystalline rocks is a brownish green.
The two highest of these hills, called Carnllidy and Penberry, are situated to the north of St. David's: they rise less abruptly from the plain than the similar hills of the neighbourhood; and are in a manner connected with each other, and with a third summit not far distant from the last mentioned, by a slightly elevated ridge which passes in a south-westerly direction from one hill to the other; Penberry being at its north-eastern extremity. From the summit of Carnllidy the ground gradually slopes towards the west for a few hundred yards, and then, again rising, forms the promontory called St. David's Head.
The ascent to the two hills above mentioned, both on the north and on the south side, is formed by highly inclined strata of a slaty rock which would be commonly called grau-wacke, a term in the present instance used only for the purpose of general description; and the nearly precipitous cliffs, by which the greater part of the adjoining coast is bounded for some miles, appear to consist principally of the same kind of rock. The massive tabular laminæ of this schist rise abruptly from the sea, with a highly elevated degree of inclination towards the land, over the edge of which they are sometimes folded in the form of a broad mantle, or are occasionally broken into natural arches and caverns; giving to the outline of the cliffs which they compose a bold but graceful curvature, very characteristic of a coast of this kind, and productive of scenery the most magnificent. A very striking illustration of this effect, though it is not clear whether it proceed from rocks of this class or from those more immediately belonging to the coal grits, occurs at Saunders' Foot, a small cove situated about four miles to the north east of Tenby.
It not unfrequently happens that the partial removal of the superincumbent laminæ of the schist, from the surface of those placed relatively beneath them, has given rise to that appearance of a succession of broad flat steps or stairs which suggested to the Swedish mineralogists the term Trapezius; a term applied by them to that class of rocks, in which from the action of the weather and other causes, there is a tendency to assume an appearance of this kind.
These slaty strata are occasionally traversed by beds of clay porphyry; and by veins of quartz affording very large and beautiful specimens of rock crystal.
It is worth noticing that in none of the stratified or unstratified rocks of this neighbourhood, did the extemporaneous test of an acid give any evidence of the presence of carbonate of lime: nor did there occur in them, with the exception of one equivocal instance, the smallest trace of any organic remains.
The rock which forms this promontory consists of a mixture of blackish green hornblende and white felspar; but the proportion of the hornblende often so far predominates, and its crystalline form is so regularly developed, that the felspar appears rather as an accidentally connecting medium of the former, than as an essentially constituent part of the whole rock. The crystals of the hornblende, though generally small, sometimes exceed two or three inches in length; in which instances they are not of a proportional breadth: in general also they are closely compacted with the body of the rock; but occasionally, and especially when larger than usual, they are easily separable from the mass, leaving a smooth impression of their surfaces. These impressions, as well as the crystals themselves, have commonly a dull iridescent semi-metallic lustre; arising, perhaps, from an increased oxidation of the iron of the hornblende, which by loosening the attachment of the crystals to the mass in which they are imbedded, has disposed the compound to assume that regularity in its fracture. A similar appearance often presents itself in parts of the Malvern rock; and it is probable that the kind of lustre here noticed is very characteristic of peculiar states of hornblende, and may serve to ascertain its presence in a compound rock where no traces of its crystalline form are evident.
The hornblende of the summit of this hill is indistinctly crystallized, and of a dark and dull olive green colour; generally very uniform in its character; and so closely compacted with the felspar that the fracture passes indiscriminately through both. The rock itself is remarkably hard, and has that degree of toughness which is characteristic of the class of rocks called by Wallerius Saxa Cornea, and Corneus Trapezius; which rocks, as may be collected from the volcanic dissertations of Dolomieu and Ferrara, contain hornblende as a principally constituent part. It occasionally contains particles of pyrites; and insensibly passes into a greyish green coarse and soft slate, which in the mass is remarkably disposed to separate into flat rhomboidal fragments, the surfaces of the laminæ of which are sometimes interspersed with a few small specks of white mica.
The rock constituting this summit, as may be satisfactorily ascertained by insensibly graduating specimens, is of the same nature with those already spoken of; though at first sight, and especially in particular parts, apparently very different. The hornblende gradually diminishing in its proportion, or being intimately blended with the substance of the felspar, often merely imparts an obscure shade of green to the whole mass; the presence of which colour principally assists the eye in recognizing the true nature of the rock; and but for which it might be confounded with a compact sandstone or felspar. The same observation holds, but still more strongly, with respect to the rock on which stands Roche Castle; a ruin situated to the north of the turnpike road, about half way between Haverfordwest and St. David's. This rock has, much more decidedly than Penberry, the character of a compact sandstone: but in its geographical relation to the surrounding country, it exactly corresponds with the preceding rocks, and with the numerous similar rocks of the neighbourhood. However this may be, the surface of the ground between Roche Castle and St. David's is scattered over with numerous large boulders, as they might be called, very closely resembling in their general character one or other of the three rocks already described; and all of them bearing strong marks of having been the result of chemical formation.
The rock of which Penberry is composed shews occasionally a slight tendency to concentric disintegration; and the external part of it is here and there altered by the action of the weather, after the manner of ferrilite.
On the coast, a little to the north-east of Penberry, is a slate quarry; worked out of a mass of schist, which forms remarkably bold and nearly perpendicular cliffs, the strata of which are occasionally much contorted. The surfaces of the laminæ of the slate have sometimes an ochry tarnish, and abound with minute particles of mica.
This quarry is situated between St. David's Head on its north, and a beach called the White Sands on its south side. The slate is of nearly a black colour, and is here and there traversed by veins of brownish white compact and indurated clay, containing tarnished cubical crystals of iron pyrites. The slate is employed in roofing; but does not thoroughly resist the action of the weather, perhaps from the effect produced on pyritical matter disseminated through it: wherefore it is customary in all this part of the country to whitewash the roof as well as the walls of their houses.
The laminæ of this slate are sometimes wrinkled or wavy. It contains but faint traces of mica; and sometimes approaches to the character of siliceous schist.
This name is given to a beach of about one-third of a mile in length, which at its northern extremity is separated from St. David's slate quarry, by a low and narrow ridge of rocks projecting into the sea; and is terminated at its southern extremity by high cliffs. Towards the land it is bounded principally by heaps of sand; interspersed with low rocky cliffs, which rise higher and higher in advancing to the southern extremity; at which point the rocks are particularly interesting from their variety; passing, sometimes abruptly, from the coarsest grained conglomerate, as from its appearance it might be called, to the finest schist.
The prevailing colours of these rocks are green and brownish purple; those colours alternating occasionally as in striped jasper. The cement of those parts which resemble a conglomerate appears to be in a great measure of chemical origin; containing minute crystals of semitransparent felspar, with small particles of glassy quartz. It seems worthy of remark, that in those parts of the rock which resemble a conglomerate, the pebble-shaped nodules of quartz are very frequently of the same purple colour as the schist. The sand of this beach when viewed through a microscope is seen to be a mixture of fragments of shells with small particles of variously coloured quartz and slate. A portion of it weighing 200 grains, which had been collected in July 1811, and had been kept in a room without a fire till March 1812, only lost one grain of its weight by exposure to a heat of 212°: after which, having been boiled in diluted muriatic acid, and then filtered, washed, and dried by the same heat, it weighed 157 grains; having lost or rather more than of its weight, which may be considered as very nearly the proportion of calcareous carbonate contained in this sand. The sand is extensively used as a manure.
This is a small fishing harbour situated to the south or south east of St. David's, and is the termination of a narrow shallow valley, which extends two or three miles inland, and is longitudinally divided by the river Alun; the mouth of the river emptying itself into this harbour.
The rocks constituting the rising ground on the left bank of the mouth of this river appear to be a compound of felspar or quartz, or both, with hornblende: the predominating colour is brownish white, arising from the great proportion of the two first mentioned component parts.
On the right bank of the mouth of the river the strata are distinctly schistose, and very various in their appearance. Within the distance of two feet the rock assumes the following characters.
Brownish purple slate, with streaks of green.
Red slate, with an incrustation between some of the laminæ resembling blackish green scaly chlorite.
Greenish grey compact sandstone.
Stratified sandy slate, partly greenish grey, partly purple.Traces of steatite and serpentine occur in the rocks of this neighbourhood.
The cathedral is situated in a part of the narrow valley through which the river Alun winds from the north east; the descent to its eastern and south eastern extremity is steep, and is formed by a rock which might be called a small grained green-stone porphyry, in a high state of disintegration.
The general colour of this rock varies between a brownish white or yellow, and a very obscurely greenish brown: where most compact it has nearly a homogeneous appearance, and would by many be described as a soft compact felspar; but commonly it is in a very loose state of aggregation.
Parts of this rock resemble Fullers' earth; but from the occasionally green colour and the peculiar direction of the natural rifts, giving it a tendency to separate into rhomboidal or into wedge-shaped fragments, it possesses a characteristic mark which serves to connect it with the prevailing rock of the neighbourhood.
Many parts of this rock easily crumble into the state of an earthy gravel, and are commonly used as a substitute for common gravel in and about St. David's.
I shall readily be excused for mentioning here, that there is in the Ashmole Museum a specimen from Jersey so very like in its general character to the part of the St. David's rock now under consideration, that even an experienced eye might be deceived as to its separate identity: and it adds to the interest of the comparison of the specimens in question, that they both occur amongst a suite of rocks composed principally of hornblende and felspar, and are both used for the same economical purpose.
To the south west of St. David's are the remains of an old entrenchment, situated near the edge of the adjacent cliff; and from the further extremity of the entrenchment the cliffs run out at right angles to the general bearing of this part of the coast, forming a tongue of land which projects into the sea. In this projecting point a natural arch exists, which has been probably excavated by the gradual washing away of part of the rock; presenting an appearance somewhat like that represented in the third of Dr. Mac Culloch's plates, in the first volume of the Society's Transactions.
Near the extremity of this tongue of land is a vein of clay porphyry of a light drab colour, containing small crystals of felspar nearly of the same colour, together with completely tarnished cubical crystals of iron pyrites: there is also in this vein an occasional appearance of decaying hornblende or chlorite. The base of this porphyritic vein wears away by the action of the weather, and leaves the crystals of the felspar projecting from the surface of the weathered part.
This vein of porphyry is inclosed in a stratum of friable schist, neither the character nor position of which are at all altered by the immediate contact of the vein. The adhesion between the schist and the vein is so very slight that it is extremely difficult, if at all possible, to separate a specimen which shall unequivocally shew the junction of the two.
The schist, which is traversed by filamentous veins of quartz, appears to the eye of a very delicately laminated structure, yet does not readily separate in the direction of the planes of the laminæ. The surfaces of many of the natural rifts have a brownish black tarnish.
The cliffs in the neighbourhood of this spot, the precise situation of which is not recollected, but it is not far distant from the foregoing, consist of highly inclined strata of indurated greenish-grey freestone; of red and coarsely laminated slaty freestone, which is used in building; and of a soft argillaceous freestone with numerous veins of sparry quartz. Traces of chlorite are very frequent in the rocks of this neighbourhood, and the slaty freestone is often interspersed with particles of a substance intermediate in its character to mica and chlorite.
Not far from Carvay I met with two insulated masses, one of which, bearing a very close resemblance to the mill-stone grit of Derbyshire, was made up of small particles of white and reddish white semitransparent quartz cemented together by white earthy felspar: the other was also a kind of grit-stone, consisting almost entirely of particles of quartz, occasionally interspersed with specks of white earthy felspar. Though I saw no rock in situ to which these masses could be directly referred, yet as they did not much differ from some of the rocks of this district, excepting in the size of their component particles, and as there was not any ground for supposing they had been brought there by art, they probably belong to the suite already described.
This is a broad headland, about three miles to the south-west of St. David's. It is frequented by trading vessels on account of a fresh-water spring which rises near the edge of the adjacent cliff's, beneath which is a convenient harbour.
The general character of the rocks in the neighbourhood of Treginnys is of that equivocal nature alluded to in the beginning of these notes: here and there they assume the appearance of a compact earthy felspar of an olive-green colour, and then probably often contain epidote, compact veins and crystals of which I saw in more than one instance. In one place this compact green rock occurs in angular columns horizontally aggregated, and forming a part of the cliffs on the north side of the harbour. The number of the columns is not above eight or ten; their form irregularly pentagonal, and their diameter less than a foot. This was the only appearance of the kind observed in the neighbourhood of St. David's.
The internal texture of many of the rocks about Treginnys, and in the road between that place and St. David's, resembles that of a very compact mechanical aggregate, the particles of which are however obscurely defined: the predominating colours are green, greenish-white and pale purple: the character of the recent fracture is like that of coarse steatite. A close inspection brings to view numerous crystalline surfaces of semitransparent laminated felspar, and particles of glassy quartz; and it is worth noticing that the felspar and quartz now and then occur in the substance of the imbedded particles as well as in the cementing medium, shewing a probably contemporaneous formation of the whole mass.
Some of the rocks of this neighbourhood approach to serpentine in their general character, and contain veins of indurated steatite.
This is by far the largest of a number of rocky islands lying off the coast of St. David's, and is separated from the main land by a channel of about two or three miles in breadth. The greatest extent of the island is from north to south, and at each extremity in this direction is an elevated summit, or beacon, of considerable height, the general character of each of which is similar to that of the corresponding summits on the main land. The intervening rocks are principally slaty. The western front of each of these two summits projects into the sea far beyond the intermediate space of land; and it appears probable that the bay interposed between the promontories has been cut out by the dashing of the waves against the crumbing schistus which is here opposed to them, and that consequently in process of time, as this schistus is extended across the island to the eastern shore, the island may eventually be separated into two.
The harbour of Ramsey is a small cove excavated in a mass of black schist, and situated near the southern extremity of the eastern front; advancing from which towards the west, and then ascending the southern beacon from its northern side, you pass over an apparently conglomerate rock made up almost entirely of large pebble shaped masses of white quartz. From the uniformity of colour in this rock, and from its general appearance, a doubt at first sight arises whether it is really a mechanical conglomerate or the result of a peculiar chemical conformation.
On and near the summit of the southern beacon the rock in many places resembles coarse chert, from which it passes into the state of indurated clay containing small crystals of felspar and of dodecahedral quartz, the latter in general very imperfectly defined.
The composition of the rock forming the northern beacon is felspar and hornblende, sometimes assuming nearly a homogeneous appearance, and sometimes, though rarely, inclining to a porphyritic structure.
With the island of Ramsey these notes on the Mineralogy of the neighbourhood of St. David's terminate, in drawing up which it has been my object to avoid as much as possible the language of hypothesis, and to detail the appearances which I met with in terms strictly descriptive: and though I feel strongly persuaded, on grounds which have not been taken up hastily, that all the rocks which I have been describing are essentially allied to each other, and are all of chemical and contemporaneous origin; yet, conceiving it would be improper on the present occasion to enter into the particulars or on the defence of those grounds, I think it respectful to the Society to be silent on those points.
I may however with propriety add, that from communications with very competent judges, aided by the inspection of specimens which they had themselves collected, it is clear to me that the geological phenomena above described are of very extensive occurrence. In Jersey and Guernsey for instance, in various parts of Devonshire and Cornwall, in North Wales and Cumberland, in the neighbourhood of Mount Sorrel in Leicestershire, in all these places severally, is found an assemblage of rocks of a decidedly crystalline character, and consisting of hornblende and felspar, associated with rocks either of a schistose structure or resembling a more or less fine-grained conglomerate, intersected not infrequently by masses of a porphyritic character, and sometimes passing into serpentine.
In the decidedly crystallized rocks of these suites, consisting of hornblende and felspar, the felspar sometimes predominates and is of a red colour, in which case the compound is usually, I believe, called sienite; but the same term seems justly applicable where the hornblende predominates and the compound is of a black or of a green colour, for the two varieties insensibly pass into each other.
The opinion which I have here ventured to express of the natural alliance between the various rocks above described is strongly supported by its correspondence with the opinion entertained by M. Godon respecting a similar class of rocks occurring in the vicinity of Boston in North America; and should it ever receive general confirmation, it might I think be fairly applied to the correction of one part of the present nomenclature of Geology. I would in that case, for instance, propose that the term sienite should be generally applicable to the whole series, and that the slaty or granular forms of it should be specifically described by those epithets; and thus, in a few instances at least, we should attain the desirable object of banishing the term grauwacke from the language of mineralogy; a term not only offensive from its harshness but still more from its want of precision.
- The transition of the natural compound of hornblende and feldspar into serpentine has been observed in Cornwall, and is satisfactorily shewn in specimens brought from thence and deposited in the Ashmole Museum; and the opinion of the natural alliance of these two rocks is strongly confirmed by the inspection of numerous specimens brought from the coast of Labrador and deposited in the same Museum.
- There is a paper on this subject by M. Godon in the 15th vol. of the Annales du Muséum d'Histoire Naturelle, p. 455, the perusal of which will amply repay those who may be induced to read it.